1. Field of the Invention
The present invention relates generally to cellular mobile communication systems in which traffic channels are dynamically assigned to each call, and more specifically to a mobile communication system which efficiently enables transmission of a broadcast message.
2. Description of the Related Art
In prior art cellular mobile communication systems, a control channel is used to transmit a broadcast announcement message for announcing that a broadcast message is being transmitted and a channel indication for notifying the identity of the channel on which the broadcast message is transmitted. In one prior art system, if a call request is issued on a traffic channel that is being used for transmitting a broadcast message, priority is on the broadcast message and the call request is rejected. Another prior art system places priority on call requests. According to this system, if a call request is placed on a base station in the presence of a broadcast message transmitted on traffic channels, an interruption message and their channel identifiers are sent to mobile stations to interrupt the transmission of the broadcast message. For example, Japanese Laid-Open Patent Specification Hei-9-214422 discloses a system in which idle traffic channels are used to transmit a broadcast message. If a call request occurs when a broadcast message is being transmitted, the broadcast transmission is interrupted and information to that effect is communicated to mobile stations on a control channel. However, the prior art systems suffer from the following problems. The first prior art cannot accept new call requests if they occur during the presence of a broadcast message since assignment of traffic channels is not performed. In the second prior art system as disclosed in the above mentioned Japanese laid-open patent specification, the transmission of the interrupted broadcast message cannot be resumed until idle traffic channels are available. In addition, broadcast messages cannot be sent in so far as idle traffic channels are not available.
It is therefore an object of the present invention to enable base stations to assign traffic channels to a call request if it occurs in the presence of a broadcast message being transmitted to continue the transmission of the broadcast message.
Another object of the present invention is to enable transmission of a broadcast message if there is no idle traffic channel.
According to a first aspect of the present invention, there is provided a broadcasting method for a mobile communication system, wherein a broadcast control channel is used to transmit identity of a channel on which a broadcast message is transmitted from a base station to mobile stations. The method comprises establishing a traffic channel, in response to a broadcast request, when at least one traffic channel is available for transmitting the broadcast message from the base station to the mobile stations, and establishing a control channel for transmitting the broadcast message from the base station to the mobile stations when all traffic channels are not available and establishing the control channel instead of the established traffic channel if a call request is placed on the traffic channel when the transmission of the broadcast message is in progress. In a preferred embodiment, the method comprises determining usage ratio of the traffic channels of the base station, and establishing a traffic channel instead of the established control channel if the usage ratio becomes smaller than a predetermined value during the time the transmission of the broadcast message is in progress.
According to a second aspect the present invention provides a broadcasting method for a base station of a mobile communication system, the base station having a plurality of traffic channels and a control channel, comprising the steps of (a) storing a received broadcast message in a memory, (b) determining whether at least one traffic channel is available and determining whether the control channel is available if all of the traffic channels are not available, (c) establishing a channel depending on channel availability of the step (b), (d) reading data from the memory and transmitting the data on the established channel, (e) if the established channel is a traffic channel and a call request is placed on the traffic channel, establishing the control channel instead of the traffic channel and performing the step (d) on remaining data in the memory, and (f) if at least one traffic channel is made available while the step (d) is being performed, establishing the available traffic channel instead of the control channel and performing the step (d) on remaining data in the memory.
According to a third aspect, the present invention provides a base station of a mobile communication system, the base station having a plurality of traffic channels and a control channel. The base station comprises a memory, a broadcast receiver for receiving a broadcast message from a network, a control processor for storing the broadcast message in the memory, determining channel availability of at least one traffic channel and channel availability of the control channel if all of the traffic channels are not available, establishing a channel depending on the channel availability of the channels, reading data from the memory, and transmitting the data on the established channel. The control processor is arranged to establish the control channel instead of the traffic channel if a call request is placed on the traffic channel during the time reading of the data from the memory is in progress, and reestablish a traffic channel instead of the control channel if at least one traffic channel is made available during the time reading of the data is still in progress.
According to a fourth aspect, the present invention provides a mobile communication system comprising a base station for establishing a traffic channel if at least one traffic channel is available in response to a broadcast request from a network, establishing a control channel for transmitting a broadcast message to mobile stations when traffic channels are not available, and establishing the control channel instead of the traffic channel for transmitting the broadcast message when a call request is placed on the traffic channel during the time the transmission of the broadcast message is in progress, the base station being in constant communication with each of the mobile stations via a broadcast control channel to transmit a command message. Each of the mobile stations is in constant communication with the base station via the broadcast control channel to receive the command message for establishing a channel to the base station in accordance with the command message.